1. Field of the Invention
The present invention relates to a focus servo controller that operates to focus a reading light beam to be irradiated on the recording surface of an optical disc.
2. Description of the Related Art
An optical disc player is provided with a focus servo controller that generates a focus error signal in accordance with the respective light levels detected by a plurality of photodetector elements of a photodetector in an optical pickup. The focus servo controller controls the position of an objective lens in the direction of the optical axis so as to decrease the level of the focus error signal, in order to focus an irradiated light beam on the recording surface of the optical disc. In the case of a four-element photodetector, two sum output signals from two pairs of photodetector elements located symmetrically with respect to the intersection of two split lines on the light-receiving surface of the photodetector are generated at individual adders. On the other hand, a focus error signal is obtained by subtracting the output signal generated by one adder from the output signal generated by the other adder. Since the focus error signal has an S-curve characteristic versus focus error, the center of the S-curve characteristic is taken as the reference level (for example, zero) of the focus error signal.
The focus error signal is supplied to a driving system to drive a focus actuator in the optical pickup, thereby displacing the objective lens in the direction of the optical axis.
The focus servo controller has an ideal control state in which the RF signal (i.e., a read signal), the sum of the output signals of the four photodetector elements, has the maximum amplitude at the reference level of the focus error signal, or the jitter of the RF signal is minimized. However, it always happens that when the focus error signal becomes the reference level, the amplitude of the RF signal is not maximized (or the jitter of the RF signal is not minimized) due to errors of optical assembly of parts including the optical pickup. For this reason, adjustment is carried out so as to maximize the amplitude of the RF signal when the focus error signal becomes the reference level.
Such adjustment are known are the bias adjustment method in which a bias voltage is applied to the focus error signal line and the balance adjustment method in which the output levels of the aforementioned two adders are adjusted to keep balance between them. In the bias adjustment method, a bias voltage is first applied in order to eliminate a direct current component or an electrical offset component in the focus error signal before the focus servo operation is initiated.
Thereafter, the focus servo system is closed to initiate the focus servo operation and the bias voltage is adjusted to maximize the amplitude of the RF signal. In the balance adjustment, a bias voltage is first applied in order to eliminate a direct current component or an electrical offset component in the focus error signal before the focus servo operation is initiated. Thereafter, the focus servo system is closed to initiate the focus servo operation and the outputs of the two adders are adjusted to balance therebetween via individual amplifiers of which gains are adjustable, in order to maximize the amplitude of the RF signal.
However, in the case where the optical pickup goes out of focus due to vibration while the optical disc is being reproduced, no output signal is generated at each of the photodetector elements, so that the bias adjustment method will cause only the bias voltage to be applied suddenly to the line of the aforementioned driving system. This results in driving the objective lens to be significantly displaced, so that a problem arises that, depending on the level of the bias voltage, the objective lens is moved toward the optical disc to collide therewith, possibly leading to damage or failure of the optical disc. Moreover, in the balance adjustment method, the objective lens can move freely since zero voltage is supplied to the driving system, and thus the vibration that caused defocusing may also cause the objective lens to move significantly and collide with the optical disc resulting in damage or failure.